Not Applicable.
Not Applicable.
1. Field of the Invention
The present invention relates to a method of treating austenitic stainless steels and articles fabricated from such steels. The present invention more particularly relates to a method of treating at least a portion of a surface of austenitic stainless steels and articles fabricated from such steels to enhance their corrosion resistance. The present invention also is directed to austenitic stainless steels and articles fabricated from such steels that are produced using the method of the invention. The invention finds application in, for example, the production of corrosion resistant strip, bars, sheets, castings, plates, tubings, and other articles from austenitic stainless steels.
2. Description of the Invention Background
The need for metals with high corrosion resistance has been addressed by the development of steels of various compositions. Articles fabricated from steels that are resistant to chloride pitting and crevice corrosion are especially important for service environments such as seawater and certain chemical processing industries. Crxe2x80x94Mo stainless steels including approximately 6% molybdenum by weight, commonly referred to as superaustenitic alloys, were developed for use in these and other aggressive environments.
Generally, the corrosion resistance of stainless steels is controlled by the chemical composition of the surface presented to the environment. Open-air annealing, a heat-treating operation commonly used in the production of stainless steels, is known to produce a chromium-depleted layer near the metal surface, under a chromium-rich oxide scale. Failure to remove both of these surfaces is known to impair the corrosion performance of stainless steels. Mechanical processes, such as grit blasting or grinding, have been employed to remove the chromium-rich scale. The chromium-depleted layer is generally removed by chemical means, namely, by acid pickling. Generally, pickling involves immersing the steel in an acidic solution, commonly an aqueous solution of nitric acid (HNO3) and hydrofluoric acid (HF), for a period of time, preferably much less than 60 minutes. To speed the pickling process the acidic solution may be at an elevated temperature, preferably a temperature at which the acidic solution is not highly volatile. It is generally known that pickling of highly corrosion-resistant stainless steels requires particular care and attention because these materials are known to pickle slowly, thereby making removal of the chromium-depleted layer difficult.
Heretofore, it has been thought desirable to pickle stainless steels using relatively dilute acid solutions. That has been the case because steel production facilities typically produce a variety of alloys, and many stainless alloys cannot withstand pickling with more aggressive pickling solutions or do not require more aggressive pickling solutions to remove the chromium depleted layer. Moreover, handling and disposing of stronger acidic solutions would require more strenuous industrial safety and environmental controls. Thus, pickling using a relatively dilute, non-aggressive, pickling solution has been used to enhance corrosion resistance of stainless steels. It has been thought that providing a stainless steel with corrosion properties that are further enhanced relative to a particular pickled stainless steel requires modifying the alloy composition. Thus, for example, increasing chromium and/or molybdenum content of a particular stainless steel has been used to improve the steel""s corrosion resistance. However, increasing the content of chromium, molybdenum, and other corrosion-enhancing alloying additions in a stainless steel increases alloying costs and may require changes to the manufacturing process. Thus, it would be desirable to provide a method of enhancing the corrosion resistance of stainless steels without modifying the chemical composition of the steels.
The present invention provides a method of enhancing the corrosion resistance of austenitic stainless steels and articles produced from the steels. The method includes removing sufficient material from at least a portion of a surface of the steel such that corrosion initiation sites present on the surface are eliminated or are reduced in number to an extent greater than has heretofore been achieved in conventional austenitic stainless steel processing. Removal of material from the steel surface may be accomplished by any known method suitable for removing material from a surface of a steel. Such methods include, for example, grit blasting, grinding, and/or acid pickling. Acid pickling, for example, occurs under conditions that are aggressive (stronger pickling solution and/or longer pickling time, for example) relative to conventional pickling conditions for the same steel. Applying the method of the invention in the production of a particular austenitic stainless steel provides corrosion resistance superior to that of a steel of the same chemical composition that has been processed in a conventional manner.
The method of the invention may provide austenitic stainless steels having a critical crevice corrosion temperature (xe2x80x9cCCCTxe2x80x9d), as defined herein, of at least around 13.5xc2x0 C. greater than steels of the same composition that have been pickled and otherwise processed in a conventional manner. For a 6% molybdenum austenitic stainless steel such as UNS N08367 (commercially available as AL-6XN(copyright) and AL-6XN PLUS(trademark) from Allegheny Ludlum Corporation, Pittsburgh, Pa.), a 13.50xc2x0 C. increase in CCCT is equivalent to at least about a 4 weight percent increase in chromium content or a 1.2 weight percent increase in molybdenum content. The method of the present invention obviates the significant increase in cost, and also the concerns over phase stability, that would be associated with such increases in alloying additive content.
The present invention, therefore, provides an economical way of significantly improving the corrosion resistance properties of austenitic stainless steels, without changing the chemical composition of the steels.